The term 'The Speed of Sound' refers to the speed at which sound waves travel through a medium, such as air or water. The speed of sound is usually measured in Mach numbers, which are named after Ernst Mach, an Austrian physicist and philosopher. The Mach number is the ratio of the speed of an object to the speed of sound in the same medium. The crossword solutions for this term are MACH (4 letters), MACHI (5 letters), and MACHONE (7 letters). These words all refer to the Mach number, which is used to measure the speed of sound.